Sandhi (/ˈsʌndi, ˈsæn-, ˈsɑːn-/; Sanskrit: संधिsaṃdhí[sən̪d̪ʱi], "joining") is a cover term for a wide variety of sound changes that occur at morpheme or word boundaries. Examples include fusion of sounds across word boundaries and the alteration of one sound depending on nearby sounds or the grammatical function of the adjacent words. Sandhi belongs to morphophonology.

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internal, at morpheme boundaries within words, such as syn- + pathy: sympathy, or

external, at word boundaries, such as the pronunciation "tem books" for ten books in some dialects of English. The linking /r/ process of some dialects of English ("I saw-r-a film" in British English) is a kind of external sandhi, as are Frenchliaison (pronunciation of usually silent final consonants of words before words beginning with vowels) and Italianraddoppiamento fonosintattico (lengthening of initial consonants of words after certain words ending in vowels).

It may be extremely common in speech, but sandhi (especially external) is typically ignored in spelling, as is the case in English, except for the distinction between a and an. Sandhi is, however, reflected in the orthography of Sanskrit, Telugu, Marathi, Pali and some other Indian languages, as with in Italian in the case of compound words with lexicalised syntactic gemination.

In Japanese phonology, sandhi is primarily exhibited in rendaku (consonant mutation from unvoiced to voiced when not word-initial, in some contexts) and conversion of つ or く (tsu, ku) to a geminate consonant (orthographically, the sokuonっ), both of which are reflected in spelling – indeed, っ symbol for gemination is morphosyntactically derived from つ, and voicing is indicated by adding two dots as in か／がka, ga, making the relation clear. It also occurs much less often in renjō (連声), where, most commonly, a terminal /n/ on one morpheme results in an /n/ (or /m/) being added to the start of the next morpheme, as in 天皇: てん ＋ おう → てんのう (ten + ō = tennō); that is also shown in the spelling (the kanji do not change, but the kana, which specify pronunciation, change).

Most tonal languages have tone sandhi in which the tones of words alter according to certain rules. An example is the behavior of tone 3 in Mandarin Chinese. When in isolation, tone 3 is often pronounced as a falling-rising tone. When a tone 3 occurs before another tone 3, however, it changes into tone 2 (a rising tone), and when it occurs before any of the other tones, it is pronounced as a low falling tone, with no rise at the end. A simple example occurs in the common greeting 你好nǐ hǎo (with two words containing underlying tone 3), normally pronounced ní hǎo.